More useful exception if no DJANGO_SETTINGS_MODULE
More useful exception if no DJANGO_SETTINGS_MODULE

Because parts of Django (in particular core.cache) access settings at
import time this can lead to confusing errors if `whitenoise.django` is
imported before `DJANGO_SETTINGS_MODULE` is defined (see e.g. #31).

This patch catches such errors and raises a more helpful exception which
explains what the underlying problem is.

A ticket has been filed against Django to remove the problematic
behaviour:
https://code.djangoproject.com/ticket/25034

hirokiky/whitenoise,evansd/whitenoise,KnockSoftware/whitenoise,evansd/whitenoise,evansd/whitenoise